Method for making magnets



Dem 31, 1963 l. s. JACOBS ETAL METHOD FOR MAKING MAGNETS Filed May 29, 1961 Num ST .1 S/ :.w w W S U f C 0 n m @K m n f wm W 5 Q m d d l* wf Gm \r\\. m m y 0. \N /J J M .w @l QN S ST. NQ. F1 Qu S l@ United States Patent O 3,116,254 METHGD FOR MAKENG MAGNET Israel S. Jacobs, Schenectady, and James S. Konvel, Albany, NSY., assignors to General Electric Company, a corporation of New York Filed May 29, 1961, Ser. No. 113,410 1 Claim. (Cl. 252-62.S)

This invention relates to permanent magnets and more particularly to permanent magnets having one easy direction of magnetization and to a method for producing such magnets.

Previously known magnetic materials have been most readily magnetized along one or more axes and have been equally readily magnetizable in either direction along each axis. The magnetic properties of these materials have been measured and evaluated by several methods, perhaps the best known being the graphical representation of the hysteresis loop obtained when a magnetic eld is applied to the magnetic material in such a manner as to cyclically reverse polarity. Since magnetic properties have been symmetrically reversible, both quantitatively and qualitatively with respect to the axis of magnetization, they also have at least two stable positions in a strong magnetic field.

While existing magnetic materials have generally proven acceptable for use in most situations, the increasing use of electronic and electrical apparatus in unusual environmental situations has resulted in a need for components having particular properties when subjected to such unusual conditions. For example, various parts of guidance or computer systems must be able to perform reliably at extreme temperatures. Development of these and other technological areas has resulted in recognition that a magnetic material having one easy direction of magnetization would be of value, particularly where this property could be utilized at low temperatures.

A principal object of this invention is to provide magnets having one easy direction of magnetization.

Another object of this invention is to provide a zinc or magnesium manganite permanent magnet with spinellike crystal structure which has one easy direction of magnetization.

An additional object of this process for producing magnets of magnetization.

Other objects and advantages of this invention will be in part obvious and in part explained by reference to the accompanying specification and drawings.

In the drawings:

FIG. l shows ahysteresis loop illustrating the magnetic characteristics of the usual type of permanent magnet;

FIG. 2 shows a hysteresis loop illustrating the magnetic characteristics of a magnet made according to the present invention;

FIG. 3 shows a shifted hysteresis loop of a magnet made according to the present invention of a slightly different composition frorn the magnet used to produce FIG. 2;

FIG. 4 shows hysteresis loops similar to the loops of FIG. 3 of materials of slightly different composition from those used in FIGS. 2 and 3; and

FIG. 5 shows hysteresis loops for the compositions used to produce FIG. 4 in which cooling in a zero magnetic ield was effected.

Generally, this invention concerns permanent magnets having one easy direction ot magnetization, which magnets comprise bodies having general compositions as indicated by the formulae:

Znxlwnt-x) [M1'12l04 (Maxl/Imax) [MnzlOi where x is greater than zero and less than one.

invention is to provide a having one easy direction Patented Dec. 3l, 1963 The process generally comprises preparing a body of the general compositions by combining the ingredients in powder form and sintering at temperatures on the order of 1150 C. or higher in air. The sintered bodies are then quenched to room temperature, and cooled in a magnetic ield to a temperature sufficiently low to create the one easy direction of magnetization within the body, this temperature normally not exceeding 30 K,

A hysteresis loop 1l) characteristic of the usual type of magnet is shown in FIG. 1, in which the magnetizing iield H, in oersteds, is the axis of the abscissa and the magnetization or M, in gauss, is the axis of the ordinate. As the iield is increased from zero to higher values of H in what for convenience may be referred to as the positive direction, the magnetization (M) of the material reaches a maximum value --Mmml for a given field. If the field -l-H is removed, the value of the magnetization decreases along the demagnetization curve 11 to MB1. If then a iield -H of reverse polarity is applied, the magnetization of the material continues to decrease along curve 11 and crosses the H axis at the value HC1, the magnetization nally reaching -Mmax as the eld is further increased in the negative direction. If the negative iield is removed, the magnetization of the material correspondingly drops to MR2, which is numerically equal to Mm. Application of the positive tield then causes the magnetization to move along the magnetization curve 12 and cross the H axis at the value Haz, which is numerically equal to HC1, As the positive field is increased, the magnetization of the material rises to the value -l-Mmx. The magnetization values M which were used in the present case in place of the flux density, B, to plot the hysteresis loops, are equal to the ilux density (B) minus the magnetizing field (H), divided by 4 pi, as indicated by the formula:

M: (B-H) /471- Compositions which can be used, according to the present invention, to produce magnets having one easy direction of magnetization have the general compositions indicated by the formulae:

where x is greater than zero and less than one. More speciiically, compositions falling within the range of from ZHo.5Mn2.'i5O4 '[0 Z110J15MT12-25O4 and Mg0-s5Mn2x15O4 t0 MgMMnmO.; have been found to be particularly useful, although compositions outside of these ranges are also effective. These materials are single phase, as viewed by X-ray, i.e., tetragonally distorted spinel, isomorphous with the mineral Hausmannite, Mn3O4. The metal ions of these materials are distributed on two different kinds of crystallographic sites-tetraheclral (A) sites and octahedral (B) sites. In these two compounds, the B-sites are nearly completely occupied by manganese ions, while the A-sites contain mixtures of the nonmagnetic zinc or magnesium ions and the balance of the magnetic manganese ions.

The displaced character of the hysteresis loops is felt to result from an interaction between ferrimagnetic or possibly ferromagnetic regions with antiferromagnetic regions. For example, the compound Mn3O4 is ferrimagnetic, while the compounds ZnMn2O4 and MgMnZO.,l are most probably antiferromagnetic. On a microscopic scale, there may be composition iluctuations in the A-sites of these mixed zinc or magnesium manganites so that some parts are more like MnO.,= and other parts more like ZnMnzO., or MgMn2O4.

Referring to FIG. 2, curve 15 illustrates the shifted loop characteristics of a body having the general composition Zn05Mn2-5O4. This body was produced by mixing zinc oxide and manganese carbonate in the correct proportions, calcining at 1000 C. in air, grinding, pressing into pellets, ring at 1150 C. in air and quenching in air. The measurements were made on powders whose particle size ranged from about 5 microns to about 150 microns (0.15 millimeter). It is believed that the state of subdivision is not critical for the observation of the displaced hysteresis loop. The body was then cooled in a magnetic field of 5000 oersteds to a temperature of 1.7 K. It is apparent that the maximum magnetizations -Mmm and +Mmaxare numerically the same, but the remanent magnetization values MR are unequal as are the coercive field strengths Hc, as shown in the following Table I:

T able I HC1 (oe.) Home.)

M m (emu.)

The hysteresis loop 16 of FIG. 3 of the drawings illustrates the magnetic properties of bodies produced in nearly the same manner as those used to produce FIG. 2, the composition in this case being Mg0'5Mn2'5O4. The only diiTerence is that magnesium carbonate was in place of zinc oxide. The magnetization and iield values of these bodies are listed in the following Table II:

Table Il Mnzmu.) Mm (emu.) HC1 (oe.) Hanne.)

Field Sample Appli ed Mmmu.) Mmmu.) Llame.) Hazme.)

Data similar to that contained in Table III was obtained from samples of the same compositions, specifically (il) ZI10 25l\/I'12 |7504= and Z110'75MI1225O4. II). this lnstance, cooling to 1.7" K was eifected without the ap- Field Sample Applged Mmmu.) Mmmu.) Heime.) Hence.)

The properties listed in Table IV clearly show that the magnetic properties are isotropic when no magnetic iield is applied during cooling.

It is felt that the present magnets as processed acquire their unique behavior by virtue of the ferrimagnetic (or ferromagnetic) and antiferromagnetic regions present within the body. The coupling eiect between these regions results in the one easy direction of magnetization. Therefore, during cooling of the body from room ternperature, it is essential, as shown by FIGS. 4 and 5 of the drawings, that it be subjected to a magnetic field, one on the order of 5000 oersteds normally being suliicient to properly orient the antiferromagnetic as well as the ferrimagnetic or ferromagnetic moments. Although the magnetic field need only be applied from just above the temperature at which the shifted loop eiTect begins, generally 30 K. or less, as a matter of expedience, it is normally easier to apply the iield continuously during the entire cooling treatment.

Although the present invention has been described in connection with preferred embodiments, it is to be understood that modications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modiiications and variations are considered to be within the purview and scope of the invention and the appended claim.

What we claim as new and desired to secure by Letters Patent of the United States is:

In the process of producing a magnet having one easy direction of magnetization, the steps comprising preparing a sintered body having a composition according to the formula selected from the group consisting of ZHo.25-0.75M112.25-2.7504 and Mge.2s-0.75M112-25-2-7504, quenching the body from sintering temperature down to room temperature, cooling the body to a temperature of about 30 K. or less and subjecting the body to a magnetic iield during cooling to develop the one easy direction of magnetization within the body.

References Cited in the file of this patent Jacobs: I. of Applied Physics, Supp. to Vol. 30, No. 4, April 19.59, pages 30lS and 3028.y 

